The glitch takes advantage of the 1.01 game patch and the
Mercedes-Benz AMG Vision Gran Turismo car, which is
included in said patch.

With 1.01 installed players can enter Vision GT and purchase the
car for one million credits. Save and exit the game. Under Game
Data Utility delete the 1.01 patch and reboot GT6,
ignoring the prompt to redownload 1.01.

The AMG Vision will now be called [2055] and has a sale price of
333,993,856 credits or similar. Your GT6 wallet caps out
at 20 million credits so that's the amount you'll be left with when
you sell the car.

Rinse and repeat for further funds.

Looking at the game's microtransaction system where seven
million in-game credits will set you back £39.99, the bug is worth
more than £100 a time. The microtransactions, according
to Shuhei Yoshida, are intended to offer "an alternative path
to busy people". With this glitch taking mere minutes to implement,
you can expect it to be patched out sharpish before it breaks the
game's economy completely.